What Does Bottle-Feeding Have to Do With Autism Risk?

As if there weren’t already enough tension between bottle-feeding and breast-feeding moms, now a researcher at the State University of New York at Albany is courting controversy by suggesting that bottle-feeding is associated with an increased risk of autism.

In actuality, it’s not bottle-feeding per se that may be linked to autism, but the absence of breast-feeding, contends evolutionary psychologist Gordon Gallup in an article published in June in the journal Medical Hypotheses.

Gallup didn’t do his own research but based his theory on data from a January study in the journal Pediatrics. That study analyzed more than 650,000 families to show that second-born children conceived within a year or less of the first child were three times more likely to have autism than second-borns conceived several years later. Gallup believes bottle-feeding may have been to blame.

“I would predict that their next oldest sibling was bottle-fed and that the reason they were conceived within a year is because of the effect that bottle-feeding has on undermining natural birth-spacing mechanisms,” says Gallup.

“Natural birth-spacing mechanisms” refers to the fact that breast-feeding causes hormonal changes that inhibit ovulation, says Gallup. So it follows that a mom who is exclusively breast-feeding has a low likelihood of conceiving again as long as her baby continues to nurse. Bottle-feeding moms, on the other hand, resume menstruating and ovulating within a few months of giving birth. They have a far greater chance of conceiving another child soon after delivery, and are therefore at far greater risk of having a child with autism.

“They now have to contend with an increased risk of autism in their second child as a consequence of having bottle-fed their first,” he says.

Not so fast, says Keely Cheslack-Postava, the postdoctoral research fellow at Columbia University who led the Pediatrics study. She says bottle-feeding likely has nothing to do with her findings. In an email, she wrote:

Our work on interpregnancy interval and autism focused on children born primarily during the 1990s, a time period of rapid increase in autism diagnoses. Because breast-feeding rates increased during this time period relative to earlier decades, the hypothesis does not appear relevant to explaining our results or the increase in autism observed during that time period.

Gallup’s hypothesis is intriguing if simplistic, says Miriam Labbok, professor of maternal and child health at the University of North Carolina at Chapel Hill and director of the Carolina Global Breastfeeding Institute. “I think there’s more than meets the eye in all of this,” says Labbok. “I’m not surprised, but it’s probably more of a proxy for something else that is probably happening.”

Indeed, Cheslack-Postava cautioned that the correlation between closely spaced births and autism found in her study was just that: a correlation. There’s no evidence to suggest that having kids further apart will prevent autism, and there may well be many other environmental or genetic factors that occur alongside closely spaced pregnancies that could affect autism risk. The disorder is exceedingly complex, with various causes and risk factors, and researchers have not identified any single driver of its development.

In any case, Labbok notes, short spacing between births is a public-health issue in its own right, aside from any tenuous connection to autism. Internationally, mortality rates jump for children conceived soon after a sibling’s birth. In poor countries, kwashiorkor — malnourishment due to protein deficiency — is common when a toddler is pushed off the breast to make way for a new baby.

But to Gallup, the association between birth spacing and autism is yet another example of what he calls evolutionary “mismatches” between our current environment and that to which we adapted over thousands of years.

Take obesity, for example. Historically, food supplies have been unreliable, with periods of famine interspersed with periods of plenty. During the plentiful eras, humans wisely stored surplus energy in the form of body fat so they would be well-equipped to survive when food was scarce. Nowadays, food is continually available, but the evolutionary drive to consume and store calories to stave off hard times endures. “As a consequence, people overindulge and end up obese,” says Gallup. “Body fat is very adaptive.”

The same concept applies to bottle-feeding, says Gallup. Mothers have nursed their babies since, well, forever. But with the advent of the baby bottle in the 1800s, bottle-feeding became increasingly popular, especially as women went off to work during World War II. Around that time and in the years after, many doctors recommended bottle-feeding.

Today, in contrast, research on the myriad benefits of both breast milk and breast-feeding bolsters advice from health organizations like the American Academy of Pediatrics to breast-feed exclusively for six months and to continue nursing for at least one year.

Yet many women who start out breast-feeding switch quickly to bottle-feeding, so even breast-feeding mothers who wean their babies at six or nine months could conceive another child within a year of the birth of their previous child.

It may be thought-provoking, but in the end, Gallup’s theory is too simplistic, as Labbok notes. And as the name of the journal publishing Gallup’s paper suggests — Medical Hypotheses — it’s pure conjecture. But theories like Gallup’s remind us just how complex a disorder autism is, and that the continuing search for its causes and risk factors bears great consequence for many parents.

Even while the bottle-versus-breast debate has no impact on your child’s autism risk, however, it might not hurt to consider Gallup’s advice, if for no other reason than to practice family planning: “Make sure you’re using contraception if you’re bottle-feeding,” he says. “Otherwise you’re putting yourself at risk of getting pregnant too soon.”

Bonnie Rochman is a reporter at TIME. Find her on Twitter at @brochman. You can also continue the discussion on TIME‘s Facebook page and on Twitter at @TIME.